Visiting Chicago

A few of my girlfriends and I are flying to Chicago for the weekend, July 9 through July 11. We're all between the ages of 21 and 25. Are there any places you all could suggest that will help us get a feel for Chicago in the couple of days we're there?

-What are some 'hot spots'?

-What sightseeing should we do?

-Is it better to take public transportation or rent a car?

-Where are good places to shop/eat/stay, etc.?

-About how much do you think we'll need for the fee? (our flight only cost $130)

Any suggestions would be lovely.